There is known a process for producing a cyclic ester, such as glycolide or lactide, by heating depolymerization of oligomer of a hydroxycarboxylic acid, such as glycolic acid or lactic acid, in the presence of a polyalkylene glycol-type depolymerization solvent (Patent document 1).
It has been also disclosed that the resultant cyclic ester is distilled together with depolymerization solvent, and the cyclic ester precipitated from the distillate liquid is separated and purified by adding a non-solvent for the cyclic ester, as desired, or the cyclic ester phase separated from the depolymerization solvent phase in the distillate is recovered by liquid-liquid separation (Patent document 1). Alternatively, there are also known a method of cyclic ester by recrystallization from an organic solvent, such as ethyl acetate (Patent document 1), and a method of washing the cyclic ester precipitated from the distillate by washing with an organic solvent, such as cyclohexanone or ethyl acetate, mutually soluble with the depolymerization solvent (Patent document 2). According to these methods of purification of cyclic esters, however, it is difficult to obtain a high-purity cyclic ester suitable as a starting material for bulk-polymerization for production of a high polymerization-degree of polyhydroxy carboxylic acid, such as polyglycolide (polyglycolic acid) or polylactide (polylactic acid). According to recrystallization from an organic solvent, it is possible to obtain a cyclic ester of high purity, but generally recrystallization is inefficient from the viewpoint of either energy or yield, and it is hard to commercially adopt it as a main method of purification of cyclic ester.
Patent document 1: WO 02/14303A1,
Patent document 2: JP-A2001-278877